February 23, 2026
4 mins read
Random Walk Theory posits that stock price changes are unpredictable, driven by new information rather than patterns. The market quickly absorbs data, making it difficult to beat using past performance or technical tools. The concept supports diversified, long-term strategies over active trading.
People who trade at the stock market look for patterns (trends) by examining past prices, price changes and news, and hope to find something that will give them insight into what will occur next. The Random Walk Theory challenges this approach to investing.
According to the theory, price movement is no different from an unexpected event. If a large volume of stock is traded, a price change occurs, and no one knows why the change happened or didn't happen.
If this theory is true, yesterday's chart provides no guarantee of obtaining the same results in the future. Thus, the upward price movement is not necessarily a reliable indicator of future price movement.
In this way, markets change from being a puzzle to being "an event". It is possible to observe the changes in the market, but it is difficult to predict those changes with certainty.
Random walk theory may sound modern, but its roots go back over a century. In 1863, Jules Regnault, a French mathematician and stockbroker, wrote about chance and the stock market. His ideas started the link between maths and markets.
Later, Louis Bachelier expanded on these ideas. In 1900, he published the "Theory of Speculation," one of the first to use statistics in finance. His work shaped how people study price behaviour.
In 1964, Paul Cootner published The Random Character of Stock Market Prices, building on this thinking. Then came Burton Malkiel’s famous book in 1973. Together, these works helped shape how we understand market movements today under the random walk theory.
Random walk theory is built on two main assumptions: first, that stock prices move randomly without following any pattern; and second, that markets are efficient, meaning all known information is already reflected in prices. Because of this, predicting future price changes using past data or analysis becomes extremely difficult.
Each price change is treated as a separate event, like flipping a coin. Just because a stock rose today doesn’t mean it will continue tomorrow. The theory says past price behaviour gives no clue about future movements.
The movement of one stock doesn’t influence another unless both are affected by the same news. According to the theory, stock prices mostly move on their own, based on their specific data or events.
The theory assumes that all available information is already priced in. So, when news comes out, the market adjusts instantly. This makes it hard to gain an advantage by using public data or analysis methods.
No one can know future news in advance. Since prices react to news, and news itself is unpredictable, price changes must also be unpredictable. That’s why the theory argues that timing the market is nearly impossible.
Additional Read: What is Chaos Theory
The random walk theory assumes that markets behave in a simple, predictable way. But real markets are more complex. Prices can react to news, but also to policy changes, interest rates, and events like insider trading.
Many traders believe price charts reveal trends and patterns. These traders use technical analysis to make decisions. They don’t agree with the idea that past prices have no role in predicting future prices.
Some investors, like Warren Buffett, have consistently outperformed the market. Their success raises questions about the theory. If markets were truly random, how could anyone beat them over many years?
The theory says all investors work with the same information. But in reality, big institutions often get better and faster data. This creates gaps that the theory doesn’t account for.
Mathematician Benoit Mandelbrot said markets don’t always move in smooth patterns. Prices can change in extreme ways. He suggested using models that include these sudden, large moves and long-term risks.
Additional Read: What is Efficient Market Hypothesis
Feature | Random Walk Theory | Technical/Fundamental Analysis |
Core Belief | Prices move randomly | Prices follow trends or reflect company data |
View on Predictability | Unpredictable; past data has no value | Predictable through charts or financial reports |
Investment Strategy Impact | Favors passive, long-term investing | Supports active trading and timing |
Price Movement Reasoning | Driven by new, random information | Influenced by past trends or financial signals . |
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Random walk theory suggests that stock prices change in a completely unpredictable way. It argues that each price movement is independent of the previous one, making it almost impossible to forecast future prices by studying past trends, charts, or patterns. In short, price changes follow no clear or consistent path.
Random walk theory and the Efficient Market Hypothesis (EMH) share a key idea: all publicly available information is already reflected in stock prices. This means new information causes immediate adjustments, making it impossible to gain an edge. The theory supports EMH’s claim that beating the market through analysis is unlikely.
According to random walk theory, consistent outperformance is rare and usually due to chance. Since stock prices follow a random path and respond instantly to new data, even skilled investors can't regularly predict or benefit from movements. The theory implies that long-term success through forecasting is not dependable or repeatable.
Critics of this theory argue that it ignores real market behaviour. They point out that some investors, like Warren Buffett, have beaten the market over time. Others say prices often follow trends, and patterns can exist. The theory also overlooks insider knowledge, uneven access to information, and external events that affect stocks.
If you consider random walk theory, the focus shifts from timing markets to building a balanced, long-term portfolio. It suggests avoiding over-reliance on technical analysis or short-term strategies. Instead, a consistent, disciplined approach that includes diversification and a long-term view is often viewed as a practical approach under this theory.
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